fix: provide an intelligible error message for multiprocessing with n…

…o config file. #1320

CHANGES.rst

@@ -23,6 +23,11 @@ Unreleased
 - Debug: added ``pybehave`` to the list of :ref:`cmd_debug` and
   :ref:`cmd_run_debug` options.
 
+- Fix: show an intelligible error message if ``--concurrency=multiprocessing``
+  is used without a configuration file.  Closes `issue 1320`_.
+
+.. _issue 1320: https://github.com/nedbat/coveragepy/issues/1320
+
 
 .. _changes_631:
 

coverage/control.py

@@ -462,6 +462,8 @@ def _init_for_start(self):
                 raise ConfigError(                      # pragma: only jython
                     "multiprocessing is not supported on this Python"
                 )
+            if self.config.config_file is None:
+                raise ConfigError("multiprocessing requires a configuration file")
             patch_multiprocessing(rcfile=self.config.config_file)
 
         dycon = self.config.dynamic_context

tests/test_concurrency.py

@@ -369,6 +369,10 @@ def test_no_multiple_light_concurrency_in_config(self):
         with pytest.raises(ConfigError, match="Conflicting concurrency settings: eventlet, gevent"):
             self.command_line("run prog.py")
 
+    def test_multiprocessing_needs_config_file(self):
+        with pytest.raises(ConfigError, match="multiprocessing requires a configuration file"):
+            self.command_line("run --concurrency=multiprocessing prog.py")
+
 
 class WithoutConcurrencyModuleTest(CoverageTest):
     """Tests of what happens if the requested concurrency isn't installed."""